Estrogen therapy improves endothelium-dependent vasodilation by enhancing nitric oxide (NO) bioactivity, which has atheroprotective implications, but is not acceptable for chronic use by many women. L-arginine may improve endothelium-dependent vasodilation by providing additional substrate for NO synthesis and thus be an acceptable substitute. We measured serum nitrogen oxides (Griess reagent technique) on a nitrate-restricted diet (as an index of endothelial NO release), brachial artery flow-mediated dilation by ultrasound following forearm ischemia (as an index of endothelial NO bioactivity), and soluble cell adhesion molecules (as an index of NO-regulated inflammatory markers) in 8 postmenopausal women at least 2 months off hormone therapies. Subjects were randomly assigned to L-arginine 9 gm or placebo daily for 1 month, with crossover to the alternate treatment after 1 month off-therapy in a double-blind study. Compared with placebo, L-arginine therapy increased serum levels of L-arginine (mean+/-SD: 74+/-16 vs. 145+/-62 micromol/L, P=0.016). However, there were no significant differences in serum nitrogen oxides (42.4+/-17.0 vs. 46.1+/-24.8 micromol/L, P=0.59), flow-mediated dilation (8.9+/-4.2 vs. 8.7+/-4.7%, P=0.91), or serum levels of E-selectin (49.0+/-17.6 vs. 49.0+/-16.3 ng/mL, P=0.97), intercellular adhesion molecule-1 (239+/-56 vs. 237+/-56 ng/mL, P=0.69), and vascular cell adhesion molecule-1 (468+/-101 vs. 463+/-67 ng/mL, P=0.82). We conclude that oral L-arginine therapy may not augment endothelial NO release or bioactivity in postmenopausal women and accordingly, is likely not of atheroprotective benefit to this population.